O.k., here is the schematic for the dual version of this filter. As you can see it is just the same filter built twice around the dual op-amp tl082.
You can run it in parallel by running a single input (same or seperate source) into each filter.
PLEASE NOTE: there are no mixers or buffers present on the outputs shown here, this leads to some very dirty crosstalk/distortion/feedback if you run both outputs to the same point. (If anyone has a relatively simple solution to share on that point please do, otherwise I'll probably build a small active mixer to sort it out...) This does yield some interesting results, but not entirely useful.
You can also run it in series, (output of the first filter into the input of the second) and this I certainly recommend, especially if you are a fan of very deep lows and some incredibly squelchy highs.
Using only a little ingenuity, you can use a DPDT switch to make it switchable between the two modes.
edit:changed to a smaller image of the schem
Personally I am running this into my PC through a Tascam US-122 interface which has two seperate audio ins with volume control for each, with fairly decent results.
Anyway, if I can be of further assistance with this for anyone, feel free to let me know.
Good luck, hope this helps,
c_z

Hi there,
I've been trying to make the filter shown here (the Sallen-Key one), and it works, but I have lots of problems with the extreme resonance. Anybody had this problem?
The cutoff dual pot works great, but the resonance one is not very useable. When I turn it up, the resonance increases but, when it begins to sound nice, suddenly it emits a very loud auto-oscillating whistle, much louder than the input signal. Is there a way to solve this changing or adding some components? Somebody here said it worked for him, so it's my filter working normally?
Thank you!

I use a 100k pot for resonance. Past the halfway point it will begin to self-oscillate. I use this so I can "max out" the squelchiness. You can use a 50k pot for the resonance, and with the components shown you should not get self-oscillation, but you will lose a small amount of resonance. I would also recommend playing around with different capacitor and resistor values to find a sound that you like.
Good luck, hope this helps.
c_z

I was building Comrade_Zero's sallen-key filter.
It doesn't really work yet, cause i haven't got a real good power supply i guess.
But while i was listening to it with my headphones on and playing with the cutoff control, I started to receive some (russian?) radio broadcast.
Does anybody know how this is possible?
I use a single 12V powersupply and tried to create a virtual ground with two resistors as voltage dividers. Is this at all possible with a circuit like this?
Using a capacitor to decouple the DC offset?

Hi, I just built the dual filter, and I think it sounds great. I'm running it off two 9V batteries. I, too, had the issue with extreme feedback with the resonance, and so on one channel, I swapped the 100k resonance pot for the top half of a dual 50k. That does indeed tame the resonance, but perhaps too much. Short of tracking down a 75k pot, is there a way to use the bottom half of the dual 50k pot and get some of the squelchiness back?

Anyhow, thanks for the schematic - it was my first successful DIY build.

Welcome aboard! Glad you got this up and running. I can't really take much credit for this circuit, as I simply tried out different values on a circuit posted by Squarewaveparade until I found some that worked for what I needed. Let me take a closer look, but you should be able to use a 100k pot and an extra resistor to limit it (your resonance pot) to 75k.
In the meantime, there are lots of great circuits floating around here. I highly recommend the sticky "A collection of circuits from Nicolas," his are more "patchable" and are great for beginners.
Good luck!
c_z

Taking a second look at it, try adjusting the value of the 33k resistor to ground on the negative input of your op-amp. If you have a trimmer (or a pot) you can "dial in" you should be able to find a value that gives you a fair amount of "squelch" without going crazy. Let us know how you get on.
Hope this helps.
c_z

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